Philips EPIQ 7 User Manual page 180

Transducers Acoustic Artifacts
Comet tail is a form of reverberation artifact produced when two or more strong reflectors are
close together and have a high propagation speed. In this case, sound does not travel directly to
a reflector and back to the transducer; and a strong linear echo appears at the reflector and
extends deeper than the reflector.
Enhancement is an increased relative amplitude of echoes caused by an intervening structure
of low attenuation.
Focal enhancement, also known as focal banding, is the increased intensity in the focal region
that appears as a brightening of the echoes on the display.
Mirror imaging artifact is most commonly seen around the diaphragm; this artifact results from
sound reflecting off another reflector and back.
Mirroring is the appearance of artifacts on a spectral display when there is improper separation
of forward and reverse signal processing channels. Consequently, strong signals from one
channel mirror into the other.
Multi-path positioning and refraction artifacts describe the situation in which the paths to and
from a reflector are different. The longer the sound takes traveling to or from a reflector, the
greater the axial error in reflector positioning (increased range). Refraction and multi-path
positioning errors are normally relatively small and contribute to general degradation of the
image rather than to gross errors in object location.
Propagation speed errors occur when the assumed value for propagation speed by the
ultrasound system is incorrect. If the actual speed is greater than that assumed, the calculated
distance to a reflector is too small, and the reflector will be displayed too far from the
transducer. Speed error can cause a structure to be displayed with incorrect size and shape.
Range ambiguity can occur when reflections are received after the next pulse is transmitted. In
ultrasound imaging, it is assumed that for each pulse produced, all reflections are received
before the next pulse is sent out. The ultrasound system calculates the distance to a reflector
from the echo arrival time assuming that all echoes were generated by the last emitted pulse.
The maximum depth to be imaged unambiguously by the system determines its maximum
pulse repetition frequency.
Reverberation is the continuing reception of a particular signal because of reverberation rather
than reflection from a particular acoustic interface. This phenomenon is analogous to the effect
created by mirrors positioned on opposite walls when an object, a head for instance, is placed
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